1. Field of the Invention
The present invention relates to a flame-rod type flame detecting apparatus for detecting flames in a pulse combustion apparatus.
2. Description of the Prior Art
With the terminology, "pulse combustion apparatus", it is intended to mean such a combustion apparatus in which flames are generated intermittently, i.e. in a pulse-like manner and which can thus be distinguished from a common combustion apparatus in which a flame of combustion is continuously present as time lapses. There are known various types of the pulse combustion apparatus. A typical one of the known apparatus comprises a valve for controlling supply of gaseous fuel and air to a combustion chamber and a discharging tube which is so designed as to produce columnar vibration (i.e. vibration of gas column) at a predetermined frequency in cooperation with the combustion chamber. In general, the combustion chamber and the discharging tube or conduit serve as a sort of a heat exchanger. In operation, a fuel-air mixture is supplied to the combustion chamber through the control valve and ignited by an ignitor to be combusted explosively. Under the pressure produced by the explosive combustion, the control valve is closed, while the gaseous combustion products are discharged by way of the exhaust or discharging pipe. Due to the discharge of the exhaust gas, a negative pressure (which means a pressure lower than that of the atmosphere) prevails in the combustion chamber, whereby the control valve is opened to suck the gaseous fuel and air into the combustion chamber. On the other hand, after-burning flame or a mass of high temperature gas present within the discharging conduit is returned to the combustion chamber under action of the columnar vibration produced through cooperation of the combustion chamber and the discharging conduit and serves as an igniting source for triggering the explosive combustion of the sucked fuel air mixture. Under the increased pressure involved by the explosive combustion, the combustion product gas is discharged. These various phases occur in one cycle which is repeated successively. In this way, the combustion takes place intermittently or in a pulse-like manner, so to say.
The frequency of the periodical pulse-like or intermittent combustion cycles lies usually within a range from 50 Hz to 80 Hz.
In connection with the combustion apparatus of the type described above, there are known various type of flame detectors for detecting the presence or absence of flames in the combustion apparatus. When flames are not in sodium yellow, and particularly in the case of the combustion apparatus for domestic use, a flame-rod type flame detecting apparatus is used. In the flame-rod type flame detector, an A.C. voltage is applied across a pair of electrodes which are disposed so as to contact with flames. When a flame is present, the A.C. voltage applied across the electrodes undergoes variations due to the rectifying action of the flame. Thus, the variations in the A.C. voltage can be taken out as a signal representing the presence or absence of flame. Detection of the presence of flame is effected during a positive half-wave period of the A.C. voltage.
Since flames produced in the pulse-like combustion apparatus described above is not in sodium yellow, the flame-rod type flame detecting apparatus described above may be used for detecting the presence or absence of flame. However, with the hitherto known flame-rod type flame detecting apparatus, the detection of flame can not be effected with a satisfactory accuracy and reliability, and often results in a failure to detect the presence or absence of flame.
As an attempt to overcome the difficulty of the rod-flame type flame detecting apparatus mentioned above, there has been proposed and used a pressure sensor type flame detector which is so designed as to detect the presence or absence of flame by detecting increase in pressure which results from the explosion brought about by the pulse-like combustion. However, the detector of this type is likely to suffer failures because a pressure sensor switch used has contact points. Further, erroneous operation is likely to take place when the combustion product gas is condensed within the pressure sensor switch, thereby deteriorating the reliability in operation.
For example, Japanese Utility Model Application Laid-open No. 123426/1979 filed Feb. 20, 1978 and laid open Aug. 29, 1979 discloses a circuit configuration of a flame-rod type flame detector circuit for detecting the presence of flame in a continuous combustion process. For the purpose of reference only, description will be made on the circuit configuration by referring to FIG. 1. In this figure, reference numeral 1 denotes a commercial A.C. power supply source to which an isolating transformer 2 having isolated primary and secondary windings is connected. The secondary winding of the transformer 2 is connected through a capacitor 3 to an electrode 4 which is adapted to be placed in a flame 5. A body of a burner or combustion apparatus 6 as well as the other end of the secondary winding is grounded. Numeral 7 denotes a load resistor. A smoothing circuit is constituted by resistors 8 and 9 and capacitors 10 and 11. The output of the smoothing circuit is coupled to a comparator 13 through a resistor bridge circuit generally denoted by 12 and constituted by resistors 15 to 18. A flame detection signal is derived at the output terminal 14 of the comparator 13 and is usually supplied to a combustion control circuit (not shown). Numeral 19 denotes a D.C. power supply source.
Description will be made as to the operation of the flame detecting apparatus of the structure described above. It is assumed at first that the flame 5 is absent. The electrode 4 is supplied with a voltage having a frequency of 50 Hz or 60 Hz from the commercial A.C. power supply 1 through the capacitor 3. Because the flame 5 is assumed to be absent, the A.C. voltage makes appearance across the resistor 7 without the waveform thereof being modified. Through the smoothing circuit constituted by the resistors 8 and 9 and the capacitors 10 and 11, the A.C. component of the voltage appearing across the resistor 7 is eliminated, resulting in that no D.C. component appears across the capacitor 11. In this connection, it should be noted that the values of the resistors 15, 16, 17 and 18 of the resistor bridge circuit 12 are so selected that the potential applied to the negative or minus input terminal of the comparator 13 is higher than the potential applied to the positive or plus input terminal thereof. As the consequence, a low level signal is produced at the output terminal 14 of the comparator 13. This low level signal thus represents the absence of flame.
Next, it is assumed that the flame 5 is present. In this case, the A.C. current supplied through the capacitor flows from the electrode 4 through the flame 5 and the burner 6 to the load resistor 7 as a rectified D.C. current due to the rectifying action of the flame 5. This D.C. current will be referred to as a detection current. As the consequence, there is produced a negative D.C. voltage component across the load resistor 7 in addition to the A.C. voltage component. When the voltage produced across the resistor 7 is smoothed by the smoothing circuit described above, a negative-going potential is produced across the capacitor 11 to lower the potential at the minus input terminal of the comparator 13. Thus, the potential at the minus input terminal of the comparator 13 is lowered below the potential at the positive or plus input terminal, so that the output signal appearing at the output terminal 14 of the comparator 13 is at a high level, representing the presence of the flame 5.